Typical refueling platforms consist of a rigid bridge crane type structure that travels on rails in the X direction between the reactor well and the spent fuel storage pool. A trolley, also a rigid structure, rides on rails on the bridge crane horizontal box section beams for motion in the Y direction. Located on the trolley are: 1) a telescoping mast and related equipment; 2) an auxiliary hoist with controls; and 3) a control station with displays. The refueling platform structure is of sufficient stiffness, the installation of the rails of sufficient accuracy, and a drive train of sufficient tightness to enable automatic grappling of the fuel based on coordinate sensors. The telescoping mast is attached to the trolley with gimbals. The mast consists of an outer heavy wall precision machined tube attached to the refueling machine at its top and four nested heavy wall precision machined tubes within. The mast has a rigidly positioned flanged adapter designed to accommodate several different end effectors. The mast extends in a manner similar to that of a telescope for a distance of about 17 m below the refueling floor.
Fuel assemblies can only be moved and replaced while the reactor is shut down. The cost of a reactor outage can be reduced by decreasing the time required for refueling. Since moving fuel is highly repetitive, it is an ideal target for automation. Although control systems for automation are readily available, refueling machines with their telescoping mast and grapple are not sufficiently free of lost motion to permit repeatable positioning accuracy and reliability needed for automation.
All current refueling machines utilize absolute positioning to achieve a predetermined repeatable Cartesian coordinate set in the horizontal (X-Y) plane. This concept has several undesirable characteristics and limitations, principally in relation to the cost of achieving the necessary position accuracy for a given depth of the fuel.
In absolute positioning, the position of the fuel in the core is assumed to have a fixed position relative to a corresponding position on the rail of the bridge or the rail of the trolley. Also a fixed invariant distance to the grapple and its intended final location is assumed. For this reason, the design must minimize deflection, minimize looseness in the mast, and eliminate lost motion in the drive trains. In a typical refueling machine with absolute positioning capability, the grapple X and Y coordinates are obtained from position indicators (typically optical digital encoders) located near the platform and trolley rails. The bridge and trolley are moved to the desired position and the mast with its grapple is lowered to the level of the fuel in the core. In this concept, deflections and clearances need to be minimized. Consequently, the bridge and trolley structures are designed to be very rigid and are therefore massive structures. Because of the large mass, a small amount of lost motion or stored energy in the drive train becomes a source of position error and it is difficult to eliminate this error source entirely. There is the inherent difficulty of precisely positioning a large mass. In addition, speed of travel becomes a problem from the standpoint of controlled nonlinear rapid acceleration and deceleration.
A separate problem is the fuel mast and grapple design. The conventional fuel mast is a series of nested tubes that extend by sliding relative to each other like a telescope. The mast must be very stiff and must be precisely machined to minimize the clearance between tubes. Consequently, the conventional mast is quite heavy. For a mast weighing 1070 kg, the estimated cable load (with fuel) is about 950 kg. For this weight, each cable must be about 9 mm in diameter and with a drum diameter of at least 405 mm (i.e., 45 times the cable diameter).
For the mast, the design challenge is to minimize clearances between the tubes and simultaneously assure smooth motion as the mast extends. If the bearing clearances are set too tight, the result is stick-slip, which is described as follows. As the mast is being extended, one of the sections is supported by the friction of the bearing and remains stuck in place. Later, as the mast continues its descent, the section breaks free and drops to the stops of the next larger mast section. Because this condition has been experienced in operating plants, the distance that a mast section may drop is limited by a load sensor that stops the descent of the mast if the full load of each mast section is not transferred as the mast is extended. Thus, the challenge is to provide a bearing design that never sticks but has zero clearance because bearing clearance introduces a non-repeatable position error.